tem. gauge hits red then returns back does not overheat
Could be a sticky thermostat. You do not want the engine to overheat. Check the coolant level. If any repair has ben done to the cooling system, then it would need to be purged from air.
And also test for intake/head gasket issues, latter likely. Compression & exhaust in coolant test.
The temperature sending unit sits in the coolant stream and is electrical. The unit can fail completely or become non responsive to the resistance within the unit. Your vehicle is 15 years old. Have you changed the the coolant regularly? Is the coolant filthy and rusty? If so, rust can clog up the sensor unit. Since the gauge swings wildly, if the problem is not with the thermostat try flushing and radiator, replace the thermostat, and the temperature sending unit. Neither part is overly pricey.
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